This invention relates generally to the art of gas lasers, and in particular to a unitary nozzle and laser channel for a supersonic electrical discharge gas laser (EDL).
In a flowing gas electrical discharge laser, a suitably conditioned gas mixture is supplied to a laser channel section where electrical discharge through the gas takes place and the laser beam is optically extracted. Gas conditioning involves control of: (1) the static pressure, which is usually less than atmospheric; (2) the static temperature, which, for example, is of the order of 200.degree. K. for CO.sub.2 laser gas mixtures, and of the order of 70.degree. K. for CO laser gas mixtures, (3) the density uniformity, which should be no greater than an order of magnitude smaller than is acceptable in standard wind tunnels; and (4) the flow velocity, which is supersonic both initially and following the discharge-lasing process.
The effect of the discharge lasing-process is to locally increase the gas temperature and pressure so that the gas is no longer usable for lasing. The flow velocity does not change, however, until relaxation waves associated with release of the local overpressure have developed, although the Mach number of the flowing gas decreases in inverse proportion to the square root of the increase in gas temperature. The flow velocity of fully relaxed gas flow is still of the same order of magnitude as the initial undisturbed flow. In operation, the electrical discharge excitation process adds a certain amount of energy per unit mass of gas. The laser power level achievable with a given sized laser channel is thus directly proportional to the gas density and the flow velocity. The use of supersonic flow to obtain expansion cooling and high power operation is more thoroughly discussed in U.S. Pat. No. 3,543,179 issued Nov. 24, 1970 to Jack Wilson, titled: Nitrogen Laser Action With Supersonic Flow, which is incorporated by reference herein.
The local increases in gas temperature and pressure caused by energy addition during the discharge-lasing process are released at sound speed (relative to the moving gas), into adjacent gas in the form of compression waves. Expansion waves are propagated back into the high pressure gas. The variation in gas density through these waves is large and these waves must thus be cleared from the discharge volume before new gas can be used for lasing. For Mach 3 flow, this has been found to require about 1.5 times as long as it takes a core flow gas particle to travel the streamwise length of the discharge volume.
The unique attractiveness of a laser as an electromagnetic wave source and what distinguishes it from other sources of optical beams is that an ideal laser beam is monochromatic and coherent; i.e., it is of one wavelength which is in phase across the wavefront. Because of this, two important effects follow: (1) all points on the wavefront can be brought to the same focus so that the beam intensity is much greater than the source intensity, and (2) the beam is highly directional with the angular spread limited by the diffraction at the exit aperture. The angular spread is, to a first approximation, equal to 2.44.lambda./D, where .lambda. is the wavelength and D is the beam diameter. The laser beam therefore maintains high intensity far from the source, which is optically referred to as the far field.
Different lasers are compared for excellence in terms of the intensity of which they are capable in the far field. It is possible for a small, low power laser to achieve higher laser beam intensities in the far field than a large, high power laser, because the far field intensity of a laser depends on the quality and design of the optical resonator cavity, the electrical-to-optical conversion efficiency and the optical quality, i.e. laser medium homogeneity, of the laser medium. For the most part, each of these three criteria is independent of the others, i.e. for a given laser the cavity optics, the gas discharge-laser excitation physics, and the laser medium optical quality can be separately optimized. However, the specific actual performance of a laser in the far field is dependent upon how well the specific device configuration meets all three criteria.
The optimization of the first two criteria above, i.e. the cavity optics and the gas discharge laser excitation physics have been extensively dealt with elsewhere. See the Wilson patent referenced above, as well as U.S. Pat. No. 3,906,392 to Mann, and U.S. Pat. No. 3,995,189 to Haslund.
The optimization of the third criteria, laser medium homogeneity, is the subject of this application. Laser medium homogeneity is a direct function of the contour of the flow channel, which extends from a point some distance upstream of the supersonic nozzle inlet to the downstream end of the discharge volume, and hence, this application concerns an optimized flow channel configuration.
The degree of laser medium homogeneity required is established quantitatively in terms of the optical path length of the laser wavefront through the gas in the laser cavity. A high performance laser holds the phase shift of the laser wavefront passing through the gas to a valve of a tenth of a wavelength or less. The relationship between optical path length and phase shift can be expressed in terms of flow density uniformity as follows: ##EQU1## where .rho. is the core flow density whose maximum variation is .DELTA..rho.; n.sub.s is the optical index of refraction at the laser wavelength at standard pressure P.sub.s (1 atm) and temperature T.sub.s (273.degree. K.); P and T are the core flow static pressure and temperature, respectively; and d is the optical path length. For a phase shift of a tenth of a wavelength (.lambda./10) in a 20CO.sub.2 -3ON.sub.2 -50He laser gas mixture for which (n.sub.s -1) is 1.97.times.10.sup.-4, P=0.75 atm, and T-200.degree. K. at a laser wavelength of 10.6.times.10.sup.-6 m, a 0.1% gas density uniformity (.rho.p/p) would permit a maximum total path length of 5.2 m. Such a path length could be made up of 10 passes of 52 cm each. Under the same conditions, a phase shift of hundredth of a wavelength (.lambda./100), a maximum path length of 52 cm would be permitted.
It is apparent that (1) the shorter the laser wavelength, (2) the greater the initial gas density (higher static pressure and lower static temperature) and (3) the greater the optical index of refraction, the greater will be the gas density uniformity requirement for a given sized laser. Large supersonic electrical discharge lasers are designed to have optical path lengths of the order of a meter. For a 10CO-20Ar-70He laser gas mixture for which (n.sub.s -1) is 1.5.times.10.sup.-4, P=0.25 atm and T=70.degree. K. at a laser wavelength of 5.3.times.10.sup.-6 m, a 0.1% gas density uniformity over a total path length of 1.0 m would produce a phase shift of .lambda./47. A 10CO-80Ar-10He laser gas mixture at the same static conditions (primarily determined by a CO content) has a value of (n.sub.s -1)=2.62.times.10.sup.-4, so that for a density uniformity of 0.1% the same path length of 1.0 m would produce a phase shift of .lambda./21. It is therefore clear that for a given size laser channel, and a given gas mixture and minimum allowable phase shift, there is a maximum gas density and, therefore, a maximum power level at which the device can operate. Any decrease in density uniformity will result in a degraded laser medium, leading to beam dispersion and decreased intensity in the far field, which is a direct loss in laser performance. Hence, a high gas density uniformity is a very important laser design objective.
Other losses can also be identified which are flow-dependent. These are associated with the development of a thermal boundary layer. The thermal boundary layer extends from the channel wall to a point in the flow at which the temperature is 0.5% larger than in the core flow. Since the static pressure is constant across the channel, the gas density varies inversely with the local static temperature and hence, the gas density profile across the flow is the inverse of the gas temperature profile. The velocity of the gas flow decreases in the vicinity of the wall. As the boundary layer flow is deaccelerated, there is a recovery of the total, or stagnation, temperature of the gas. A velocity boundary layer is also defined, covering the distance from the wall to a point in the flow where the velocity of the gas is 1% smaller than the core flow velocity. The thermal boundary layer is typically 10%-30% thicker than the velocity boundary layer.
The existence of the low density thermal boundary layer presents two sources of losses in the performance of a supersonic electrical discharge laser. First, the useful discharge cavity height for a given channel height is reduced due to thermal boundary layer growth on the channel walls parallel to the optical path of the wavefront in the discharge cavity, as any discharge energy added to the thermal boundary is wasted. Second, the electrical discharge must be confined well within the bounds of the discharge cavity in the direction of discharge to prevent breakdown (arcing) through the low density gas.
The second constraint also leads to the need for an electrical stand-off distance between the edges of the high voltage discharge electrodes and the nearest grounded conductor. For a metal nozzle, this imposes a streamwise stand-off distance requirement, which necessarily results in an increase in the thermal boundary layer thickness in the vicinity of the electrodes, with resulting large performance losses of the first kind noted above.
The minimum electrical streamwise stand-off distance is roughtly equal to the temperature ratio across the nozzle, (i.e., before and after supersonic expansion) multiplied by the electrode separation distance (channel height). The temperature ratio is used as a multiplying factor because the gas at rest at the channel wall surface has a temperature on the order of the initial gas total temperature, and the ratio of wall to core flow gas temperature is equal to the ratio of core flow to wall gas density. For a Mach 3 10CO-20Ar-70He gas laser, for example, the minimum stand-off distance is equal to 3.75 times the electrode gap width.
It is apparent that a thin thermal boundary layer is desirable to minimize device-associated losses. It has also been found that for a given electrode gap width a thin thermal boundary layer permits both higher voltage operation without breakdown and higher current densities at the higher voltages. This leads to higher maximum input power densities and improved laser performance. It has been demonstrated by one embodiment of the subject invention description that a reduction of thermal boundary layer thickness from 25% to 15% of the fixed channel height increased the input power density capability by 36%.
The thickness of the thermal boundary is directly dependent on the length of the nozzle laser channel. Thus, short nozzle laser channels are desirable. To date, however, long nozzle laser channels have been required in order to provide the required electrical isolation between the electrodes and the metal nozzle. A metal nozzle (usually hardened steel) has been used in order to satisfy the very high precision wall contour tolerances. The metal nozzle and laser channel have been made separately with non-optimized contours and flanged together with small steps at the flange interface exceeding the minimum step size, which is 0.0001 inch for Mach 3 flow, for example. Consequently, a steady state wave system with density nonuniformities was propagated into the discharge-cavity region, degrading the laser medium.
Taking the above considerations in combination, it is apparent that conventional electrical discharge gas lasers operate significantly below their inherent capability, due to the use of relatively low flow densities, unsuitable nozzle and laser channel configuration and construction, and relatively thick thermal boundaries in the discharge region.
Accordingly, it is a general object of the present invention to provide a combined nozzle-laser channel for use in a supersonic electrical discharge laser which overcomes one or more of the disadvantages of the prior art stated above.
One object of the invention is to provide such a device wherein the nozzle and the laser channel are unitary.
An additional object of the present invention is to provide such a device which minimizes flow dependent losses in a gas laser.
Another object of the present invention is to provide such a device wherein the density of the gas flow in the laser channel is uniform to the order of 0.1% or better.
A further object of the present invention is to decrease the thickness of the thermal boundary layer in the region of the main discharge electrodes.
Yet another object of the present invention is to increase the power capability of gas lasers.
A still further object of the present invention is to increase the efficiency of gas lasers.